1. Field of the Invention
The present invention relates to a typical oxide magnetic material having soft magnetism, and more particularly to an Mn—Zn ferrite suitable for low-loss materials for use in switching power supplies, various inductance elements, impedance elements for EMI countermeasures, electromagnetic wave absorbers, and so forth.
2. Description of the Related Art
An Mn—Zn ferrite is one of typical oxide magnetic materials having soft magnetism, generally contains base components: more than stoichiometric composition of 50.0 mol %, average 52.0 to 55.0 mol % Fe2O3; 10.0 to 24.0 mol % ZnO; and the remainder consisting of MnO, and is usually manufactured such that raw material powders of Fe2O3, ZnO and MnO are weighed for a predetermined compound ratio, mixed, calcined, milled, component-adjusted, granulated, and pressed into green compacts having a predetermined shape, and the green compacts are sintered at a temperature of 1200 to 1400 degrees C. for 2 to 4 hours in a reducing atmosphere with partial pressure of oxygen controlled by charging nitrogen in a furnace in accordance with a formula (1) described below, and are cooled down in the same atmosphere;log Po2=−14540/(T+273)+b  (1)where T is temperature (degrees C.), Po2 is partial pressure of oxygen, and b is a constant (usually set at 7 to 8).
It is generally known that manganese (Mn) component of an Mn—Zn ferrite can be present as Mn3+ or Mn2+, an abundance ratio between Mn3+ and Mn2+ depends on atmospheric partial pressure of oxygen at sintering, and that Mn3+ deteriorates soft magnetism of an Mn—Zn ferrite significantly. It is also known that an electron transfer, which occurs between Mn3+ and Mn2+, causes electrical resistance to lower. Accordingly, in order to manufacture an Mn—Zn ferrite with an excellent soft magnetism and a high electrical resistance, it is essential to control a sintering atmosphere (partial pressure of oxygen) so as to minimize the production of Mn3+, and the constant b of the aforementioned formula (1) is set at 7 to 8 in consideration of the essentially together with industrial feasibility. The fact that the constant b is set at 7 to 8 means the partial pressure of oxygen at sintering must be controlled within a narrow range, which makes the sintering process very troublesome pushing up production cost.
In the conventional general Mn—Zn ferrite containing more than 50.0 mol % Fe2O3, iron (Fe) component can be present as Fe3+or Fe2+, and when the Mn—Zn ferrite is sintered in the reducing atmosphere described above, Fe3+is partly reduced to produce Fe2+. Fe2+ has a positive crystal magnetic anisotropy and cancels out a negative crystal magnetic anisotropy of Fe3+ thereby enhancing the soft magnetism, but an electron transfer occurs, like the manganese (Mn) component, between Fe3+ and Fe2+ thus lowering electrical resistance significantly.
Recently, with more and more electronics equipments coming out in a reduced size with a higher performance, processing signals are formed with a higher frequency, and therefore magnetic materials are required to exhibit excellent magnetic characteristics in a high frequency band. With a magnetic core formed of an Mn—Zn ferrite, an eddy current flows increasingly at a higher frequency thereby increasing a loss. Consequently, its electrical resistance (resistivity) needs to be maximized in order to enable the Mn—Zn ferrite to duly function as a magnetic core material in the highest frequency band possible. However, since the conventional Mn—Zn ferrite contains more than 50 mol % (stoichiometric composition) Fe2O3, Fe2+ is present in a large amount, which facilitates an electron transfer between Fe3+ and Fe2+ (ions). As a result, the resistivity is smaller than an order of about 1 Ωm, and therefore the Mn—Zn ferrite can duly function only up to several hundred kHz of frequency, from which upward the initial permeability is significantly lowered whereby its characteristics as a soft magnetic material is totally lost.
Under the circumstances, Japanese Patent Applications Laid-Open Nos. H07-230909 and H10-208926 disclose an Mn—Zn ferrite containing less than 50.0 mol % Fe2O3, and also containing CaO and SiO2 as additive in order to increase electrical resistance, and Japanese Patent No. 3108803 discloses another Mn—Zn ferrite containing less than 50.0 mol % Fe2O3, and TiO2 or SnO2, and also containing CaO and SiO2 as additive in order to increase electrical resistance.
The Mn—Zn ferrite disclosed in the aforementioned Japanese Patent Applications Laid-Open Nos. H07-230909 and H10-28926 is destined for a magnetic core material of a deflection yoke and therefore is intended to be used only up to 100 kHz of frequency (refer to the embodiments described in the Japanese Patent Applications), and it is not assured that the Mn—Zn ferrite can generate excellent magnetic characteristics (soft magnetism) in a high frequency band exceeding 1 MHz. Consequently, the Mn—Zn ferrite cannot function successfully as a magnetic core material in a high frequency band exceeding 1 MHz. The aforementioned Japanese Patent Application Laid-open No. H07-230909 indicates that the Mn—Zn ferrite can contain up to 0.50 wt % CaO and SiO2, but the examples discussed therein contain less than 0.10 wt % CaO thus none of the examples contain more than 0.20 mass % CaO. And it is described therein that Mn2O3 may be added in an amount adapted to make a total of about 50.0 mol % together with Fe2O3, but since the Mn—Zn ferrite contains 45.0 to 48.5 mol % Fe2O3, 1.4 to 5.0 mol % Mn2O3 (i.e. Mn3+) is to be added to make 50.0 mol %. If such a large amount of Mn3+ is contained, it is difficult for the Mn—Zn ferrite to satisfy the requirements of both the soft magnetism and the electrical resistance.
On the other hand, the Mn—Zn ferrite disclosed in the aforementioned Japanese Patent No. 3108803 contains small amounts of CaO and SiO2, specifically 0.005 to 0.20 mass % CaO and 0.005 to 0.05 mass % SiO2, and therefore is not quite effective in increasing the electrical resistance. If CaO and SiO2 contents are increased trying to increase the electrical resistance, an abnormal grain growth occurs at sintering and the magnetic characteristics (soft magnetism) is significantly deteriorated.